In the manufacture of iron cores, e.g., for motors, transformers and the like, it is known to provide electric sheet steel with an insulating layer, subsequently to blank the sheet steel, to stack the blanks and to join them by welding at their edges. The insulating layer which has been applied has a certain insulation resistance and has a substantial influence on the edge life of the blanking tools, i.e., on the time after which the tool must be re-sharpened. Important criteria for the quality of the insulating layer are its behavior during the stress-relieving annealing, which is required in most cases, and its influence on the seam weld formed as the blanks are welded.
Such insulating materials may be inorganic and may be formed, e.g., by means of treating liquors which contain chromic acid and/or phosphoric acid and/or phosphates. Whereas such layers usually have a satisfactory insulating resistance, the wear of the blanking tools will generally be relatively high in such cases and the use of chromic acid is not desirable from an ecological aspect.
Another kind of insulating layers are formed by an application of treating liquors which contain organic resins and optional inorganic additives and will, in many cases, result in a longer edge life of the tools; as a rule, however, their bond strength after the stress-relieving annealing and their influence on the formation of the seam weld are unsatisfactory.
It is an object of the invention to provide for the application of an electrically insulating coating on sheet steel a process which is free of the disadvantages of the above-mentioned processes and which can be used to form insulating layers which are satisfactory in every respect whereas additional process and equipment expenditures are not involved.